Automatic frequency control



S pt. 16, 1941.

J. D. BRAILSFORD AUTOMATIC FREQUENCY CONTROL Filed Sept. 2, 1938 2Sheets-Sheet l Fig.1

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P 16, 1941' J.-D.VBRAILSFORD 2,255,746

AUTOMATIC FREQUENCY CONTROL Filed Sept. 2, 1938 2 Sheets-Sheet 2 ToLOCAL W OSCILLATOR g] COMPENSATING TANK C/RCU/T 3 L/ BIAS 0 AFC 514.0"

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JOSEPH 0. BRA/LSFORD ATTORNEY. I

Patented Sept. 16, 1941 AUTOMATIC FREQUENCY CONTROL J ph D a B il f rLondon, En and s.- signor to Radio Corporation of America, a cor- Ioration of elaware Application September 2, 1938, Serial No. 2 555,131";I Great Britain ct ber 5, 19.

8 C aims.

This invention relates to radio receivers, and more particularly toautomatic frequency con-. trol systems (A. F. C.) for use insuperheterodyne receivers. I

By the expression A. F. C. system, as herein employed, is meant a systemwhereby the tuning of a receiver as a whole is automatically main!tained correct for a station to be received when once the said tuninghas been set manually within a predetermined short distance of thecorrect tuning position for that station. A. E. C. systems are sometimesrather loosely termed automatic tuning systems since a measure ofautomatic tuning is obtained thereby, although in fact an A. F. C.system provides merely accuracy of tuning, selection between stationsbeing effected by the usual manually variabletuning control means.

Superheterodyne receivers with A, F. 0. system are now well known and incommon use. In general, such systems operate by taking; energy from someconvenient intermediate frequency point of the receiver, passing itthrougha discriminator network which is frequencyseleetive, and.utilizing the discriminator networkoutput to actuate an auxiliarycontrol device which exercises a measure of control upon the localoscillator frequency, the arrangement being such that so long as theintermediate frequency energy actually fed to the discriminator networkis the predetermined intermediate frequency of the receiver no controlis exercised, but (assuming the receiver is first manually tuned withina predetermined small distance of the correct tuning position for thestation to be received) if the intermediate frequency energy fed to thediscriminator network departs from the predetermined intermediatefrequency the output from the said network actuates the auxiliarycontrol device to vary the local oscillator frequency to such an extent,and in such a sense, as to bring the intermediate frequency actuallyproduced into substantial coincidence with the predeter= mined value. Inthe usual type of arrangement the output from the discriminator networkis rectified to produce a control bias which is applied to the controlgrid of a control valve whose anode-cathode space is included in acircuit which is connected across, and forms Part of, the frequencydetermining circuit of the local oscillator so that the control valveacts as a variable reactance dependent in value upon the bias appliedthereto, and, therefore, in turn de-, pendent upon the discriminatornetwork output,

Now, it is very desirable that the range and law of control of an A, F.C. system in a superheterodyne receiver shall be substantially constantthroughout the tuning range of the re, ceiver, and a defect of known A.E. C. systems is that this desirable result is not achieved, the rangeand law of control varying throughout the tuning range of the receiverin dependence upon, the setting of the manual tuning control. It hasbeen sough t r ce this. d f by pr viding an inductance across thegrid-cathode circuit of the control valve. This inductance is,therefore, in parallel With the eifective condenser which is responsivefor the phas changing action and has the effect of providing a circuitwhich, while acting as a capacity over the range of frequenciesconcerned, has an impedanceefrequency law such that a correcting actiontakes place. Although this provision doees undoubtedly reduce thevariation of range and law of control, it still leaves an undesirablylarge amount of variation over the tuning range.

The. principal object of the present invention s to. reduc or avo th sThe dev a io o k n F. C- si ems r m the ideal constant control is mostmarked on the h ghe f eq enc n band and it s. herefo e on hes b nds hahe nventio can be a p ie i h m st ad anta e. The said nvention s hwever, of general appl at on, nd s not limited to its u e on anyparticular wave band- According to this invention a superheterodyneradio. receiver is characterized in that control for A. F. C. action isexercised not only in dependence upon departures from correct tuning,but also in dependence upon the setting of the manual tuning controlmeans for the receiver as a whole, the second mentioned control being asupplementary, or compensating, control operating to insure that therange and degree of A. C. action shall be substantially the samethroughout the tuni g range of the receiver. It will thus be seen thatthe characteristic feature of the present invention resides in the usein combination of (1) the hitherto usual type of 11 19 d n c pon deartures from a correct tuning PQSition, and (2) the supplemena s! ontrolfo mpensatin purposes, thi control operating automatically in dependenceupon the setting of the manual tuning control means i he re eiver.

The present invention may be employed alone, or in conjunction with theknown expedient 1 resistance R2 by earthed screens.

1 T2 but this'is of negligible consequence. important and desired effectof the screening is to reduce the capacity across 32.. 1 Fig. 2 shows afurther modification which 1 differs from Fig. 1 mainly in that'areactive'im- 1 pedance X, provided for the purpose of-correcting abovereferred to of providing an inductance across the grid circuit of thecontrol valve.

In the drawings:

Fig. 1 shows one form of the invention,

Fig. 2 illustrates an alternative wherein the frequency control tube ofFig. 1 includes a phase shift correction reactiv impedance,

Fig. 3 graphically shows the'eifect of the phase shift correctionimpedance of Fig. 2, Fig. 4 shows another embodiment,

Fig. 5 shows a manner of securing compen"-" sating bias, g

Fig. 6 illustrates still another modification,

Fig. 7 shows an alternative embodiment using j the arrangements of Figs.5 and. 6'.

Referring to Fig. 1, which shows diagram- 7 matically one way ofcarrying out the invention,

use is made as the control valve or variable reactance valve of a valveVl-as shown, a pen- 1 tode-having at least two grids one of which GIreceives through resistance RI main control. bias obtained from arectifier (not shown) fed from a suitable discriminator network (notshown) as in the usual way, and another of which G3 receives over lead Wcompensating bias applied in accordance with this invention anddepending upon the setting of the manually variable control means of thereceiver. G2 is a screen grid. The anode-cathode space of the valve VIis connected at Tl, T2, across the frequency determin-' ing circuit (notshown) of the local oscillator nals T1T2, the value of the reactancevarying with the mutual conductance of the valve.

In the description of Fig. 1 so far given'it has been assumed that R2 isa pure ohmic resistance and-C a pure reactance. In practice, of course,this assumption is inaccurate and there will be a stray capacity inshunt with R2 while, of course, the condenser C is shunted by the gridimpedance of the valve VI and various auxiliary components Thesedepartures from the theoretical ideal constitute causes of troubleswhich may, especially 7 at higher frequencies and where a big range of lcontrol is desired, become serious. Such difii.

culties may, howevenbe largely avoided where necessary, by screening thevalve VI and the ing is represented in chain lines in Fig. 1 where it isshown as constituted by a metallic screening tube ST through which theresistance R2, passes, and which itself passes through, and is connectedto, an earthed metalplate EPl and a metallic screening tube VT for thevalve V1, the tube VT 1 similarly passing through, and being connected 1to, an earthed metal plate EP2. the screening is'to increase theefiective capacity An effect of at C and also the straycapacity betweenT1 and The Such screeni cathode voltage) for undesired phase shift, isinserted in the cathode leg of the valve V1. The impedance X may be aninductance, or a capacity (with a choke across it to pass the D. C.) ora combination of either with resistance. The effect of the impedance Xis illustrated vectorially in Fig. 3. Assume that after the phasecorrection has been made by means of the cathode impedance X the anodecurrent OI (the vector of reference in Fig. 3) is in quadrature with thevoltage across the input terminals T1T2. thevoltage between G! and K.The supply volt- Then OE represents age OS must, by assumption, be inquadrature. The voltage between G! and earth will lead OS by nearly 90and be smaller. 0G represents this voltage. OE is the resultant of 0Gand the cathode voltage reversed in sign (minus the If X is a pureinductance the voltage across it will lead the current OI by 90 andtherefore minus the cathode voltage-this is represented by the vectorOCwill lead OI by 90. 00 must be so dimensioned that OE is the resultantof 0G and 0C for the original assumptionnamely that OI and OS are inquadratureto hold good. This requirement may clearlybe satisfied bysuitably selecting the dimensions of X.

In amodification (not illustrated) the compensating bias is applied tothe first control grid G] and the main control bias to the third gridG3.

In the embodiment illustrated in Fig. 4 the.

required compensating action is obtained by utilizing an auxiliary valveV2 automatically to vary feedback for a control valve Vi which, asbefore, constitutes a variable rea'ctance in the local oscillatorfrequency determining circuit. Here, as before, a pentode V1 is used asthe control valve and the anode-cathode space A-K of I ance R and asecond condenser C; the junction point of the elements R2 and C beingconnected to the grid G1 as before. The main control bias obtained. fromthe discriminator network and associated rectifier (not shown) isapplied r 3 througha resistance Rito the grid G1; the sec ond gridG2acts as a screen grid; and the third grid G3 is connected to thecathodeK to act as a suppressor. The cathode K is capacity coupled bya'condenser C3 to the anode A2 of the auxiliary 1 valve V2, and the gridG1 is, capacity coupled by td'earth. With this arrangement the cathodeleg resistance Raproduces negative feedback due to the control valveanode current and positive feedback due to the auxiliary valve. Thussince the compensating bias controls themutual conductance of theauxiliary valve V2 the total feedback produced 'will depend upon thecompensating-bias.

In-the foregoing description the-method of obtaining the compensatingbias has not been set forth, but obviously there are various different.

This auxiliary valve receives a circuits which can be used for thispurpose. For example the compensating bias could be derived from agraded potentiometer gang-controlled with the manual tuning condenser.The tuning condenser law of most modern receivers is such that anordinary logarithmically graded potentiometer would suflice for thispurpose. Again a rectifier fed through a suitable frequency dependent orshaping network from across the frequency determining circuit of thelocal oscillator could be utilized to provide the compensating bias. Forthe case where the voltage across the frequency determining circuit ofthe local oscillator is constant or sufliciently nearly constant,excellent results may be obtained, as shown in Fig. 5, by deriving thecompensating bias from across a capacity shunted resistance combinationI, 2, in the cathode leg of a diode 3 which is in series in a loopcircuit with said resistance 2 and with a (preferably adjustable)condenser 4, the said condenser being connected at the terminal remotefrom the diode anode to the earthy side T2 of the frequency determiningcircuit (not shown) of the local oscillator and being connected at itsother terminal through a resistance 5 to the other side T1 of the saidfrequency determining circuit.

The invention is not limited to the use of a single valve controlcircuit for other arrangements are possible. For example, as shown inFig. 6, the main control bias may be applied through a suitableresistance R'i to the control grid Gi of one control valve Vi saidcontrol grid being connected through a condenser C5 to earth and to theanode A" of a second control valve Vi to Whose control grid G"1 thecompensating bias is applied through a suitable resistance R."4.. Thecathodes KK" of the two control valves are connected together and to theearthy side of the frequency determining circuit (not shown) of thelocal oscillator, the control grid G"1 of the second valve V1 beingconnected both to the other side of said predetermined frequency circuitand also to the anode A of the first control valve V'i. Any knownconvenient means are provided for applying static bias potentials to thevarious valve electrodes but it is not thought necessary to describesuch means herein.

In Fig. 6 the anode and grid static biasing arrangements have beenomitted for the sake of clarity. t is, therefore, a diagrammaticrepresentation of a two-valve circuit for producing a reactance byelectronic means. The physical reactance C5 causes the voltage fed tothe grid G'i to be in quadrature with the anode current of the valveV"1. Thus, the anode current of V'.1 is in quadrature with the anodecurrent of V"'-1 and, therefore, with the grid voltage of V"i. It willbe seen, therefore, that the application of an alternating voltage tothe terminals Ti-T2 results in a current between these terminals whichis in quadrature with the applied voltage. Fig. 6 is equivalent,therefore, to a reactance connected between the terminals Ti--Tz. Thevalue of this reactance can be shown by analysis to be dependent uponthe product glllgl1 9"1 and gi being the respective mutual conductancesof valves V"1 and V1. The fact that the compensating bias is applied tothe valve V"1 is simply to illustrate this possibility, as it will beseen from the aforesaid expression that variation of any of the factorsC5, g"1 or g'i will vary the v 4 will fall as the frequency rises.

apparent reactance between the terminals TlT2 and, therefore, afiectthecontrol. The terminals T1-T2 are, of course, connected to the oscillatorcircuit as in the case of the previous figures.

In a modification shown in Fig. '7 of the last described arrangement thesecond control valve V"i is also employed as the local oscillator of thereceiver. In this modified arrangement the main control bias is appliedthrough a suitable resistance R'i to the third grid Ga of a pentode V'iwhich constitutes the first control valve, and the compensating bias isapplied through a suitable resistance R'4. to the first control grid Giof this valve, the second grid Gz acting as a screen grid. The valvereceives automatic bias by means of the usual capacity shuntedresistance combination BR in its cathode leg, and its anode-cathodecircuit includes in addition to this self-bias resistance a blockingcondenser BC and the manually variable frequency determining circuit TCOof the local oscillator. The control grid G'i of this first controlvalve is connected through a condenser to earth and is capacity coupledto the anode A of a second control valve V"1 (also a pentode) Whosefirst control grid G"1 is connected to its cathode K" through aresistance R"4. This resistance is shunted by a condenser C63 in serieswith a portion of the inductance of the frequency determining circuitTCO. The second grid G2 of the second control valve acts as a screengrid, and the third grid G"3 thereof is connected to the cathode K toact as a suppressor grid. Both valves receive anode potentials throughsuitable chokes CH CH. Voltage is taken from the frequency determiningcircuit TCO from across a portion of the inductance thereof to a diodenetwork which supplies the compensating bias. This network is as shownin Fig. 5. This arrangement is particularly suitable for use on shortwaves.

Fig. 7 is complicated by the fact that the valve V"1 acts in the dualrole of oscillator and the first valve of the arrangement of Fig. 6. Theposition of the terminals Tl--T2 will make this clear. Because of theresistance-capacity combination 5-4, the voltage across the condenser beshown that the control effect is proportional to the product offrequency times effective mutual conductance of the compensating valve(or in the case of Fig. 6 compensating valves). In

V order that the control effect should be constant the effective mutualconductance must be inversely proportional to frequency; that is, as thefrequency rises the effective mutual conductance must fall. It will beevident, therefore, that since 1 the voltage across the condenser 4falls as frequency rises, the positive compensating bias applied to G'idecreases and the mutual conductance falls, thus fulfilling the requiredcondition. Variation of the tuning of T. C. 0. will, of course, changethe frequency. Now, the bias over W is derived from rectification of thehigh frequency voltage across the condenser. This condenser is in turnfed through the resistance 5. The impedance of 4 varies with frequency,and, therefore, by potentiometer action the voltage across 4 varies withfrequency. Correct dimensioning of 4 and 5 results in a variation in thebias W which is a very close approximation to that recs uilred forcomplete compensation of the con- The invention is not limited to theparticular Now it can circuits above described and. variousmodifications may be made. In particular the network by means of whichthe compensating bias is obtained need not be as above specificallydescribed but may take any of a variety of filter or frequency dependentforms known per se in dependence upon the requirements in any particularcase as to the law of compensation.

What is claimed is:

1. In combination with a tunable oscillator circuit, means for tuningthe circuit over a range of frequencies, an electron discharge tubehaving its plate to cathode impedance connected across the oscillatorcircuit to provide a supplemental tuning element, means for varying thegain'of the tube to vary the magnitude of the said impedance, andauxiliary means, responsive to adjustment of said tuning means over saidrange, for varying the gain of said tube supplementally'of the firstmeans with voltage derived from said. oscillator circuit, said tubeincluding at least two control grids between'its plate and cathode, saidfirst'm'eans varying the voltage of one grid, and said. auxiliary meansvarying the voltage'of the second grid.

2. In combination with a tank circuit of an oscillator adapted to betuned over a wide range of frequencies, a tube having at least acathode, anode and two control grids, means connecting the anode tocathode. path of the tube across the tank circuit thereby to provide areactive.

effect across the latter, means for varying the V voltage of one grid toadjust the magnitude of said effect, and means for supplementallyvarying the voltage of the second grid with voltage derived from thetank circuit to maintain said magnitude adjustment according to apredetermined laW of variation, and both said varying means consistingof sources of direct current voltageof variable magnitude. H 4. Incombination with the tank circuit of an oscillatoradapted to be tunedover a Wide range of frequencies, a tube having at least a cathode,

anode and two control grids, means connecting the anode to cathode pathof the tube across the tank circuit therebyv to provide a reactiveefiect across the latter, means for varying the voltage of'one grid toadjust the magnitude of said ef.-

feet, and means for supplementally varying the voltage of the secondgrid to maintain said magnitude adjustment according to a predeterminedlaw of variation, said supplemental varying means comprisinga rectifiercoupled to said tank circuit to derivedirectcurrent. voltage fromoscillations, and means for applying the derived voltage to the secondgrid. v V

5. A radio receiver having manually controlled tuning means forselecting any one of a number of different carriers within a range offrequencies, electronic meansact uated by any inaccuracy in theoperation of said tuning means which substantially compensates therefor,and a source of voltage derived from high frequency energy, Whosemagnitude is dependent on the tuning means adjustment, for controllingsaid electronic means to maintain the ratio ofsaid compensa: tion tosaid inaccuracy uniform over said range,

6; In combination with a tunable oscillator circuit, means for tuningthe circuit over a range of frequencies, an electron discharge tubehaving its plate to cathode impedance connected across the'oscilla'torcircuit to provide a supplemental tuning element, means for varying thegain of the tube to vary the magnitude of the said impedance, andauxiliary means, responsive to adjustment of said tuning means over saidrange, for varying the gain of said tube supplementally of the firstmeans with voltage derived from said comprising a rectifier coupled tosaid oscillator circuit to derive a direct current voltage fromoscillations produced in the oscillator circuit, and tlnaans forapplying the derived voltage to said '8. In combination with a tunableoscillator circuit, means for tuning the circuit over a range offrequencies, an electron discharge tube having its plate to cathodeimpedance connected across the oscillator circuit to' provide asupplemental tuning element, means for varying the gain of the tube tovary the magnitude of the said impedance, and auxiliary means,responsive to ad justment of said tuning means over said range, forvarying the gain of said tube supplementally of the first means withvoltage derived from said oscillator circuit, said oscillator circuitincluding a tube, means coupling the plate circuit of the oscillationtube and an input electrode of said first tube, and said auxiliary meansbeing connected to said input electrode to vary the voltage thereof.

JOSEPH DOUGLAS BRAILSFORD.

